One of the environmental concerns recently focused on is the destruction of the ozone layer. What influences are caused by the destruction on our life? The life of biological organisms on the earth is supported by sunlight, and simultaneously, hazardous ultraviolet light in sunlight constantly threatens the life. Ultraviolet light is a part of sunlight at a wavelength of 100 to 400 nm and is largely divided into three parts, namely UV-C at 100 to 290 nm, UV-B at 290 to 320 nm and UV-A at 320 to 400 nm. Specifically, ultraviolet light at a wavelength of 320 nm or less is absorbed in the ozone layer, while ultraviolet light at the other wavelengths, namely a part of UV-B and UV-A pour on the earth. Particularly, an ultraviolet light component at a wavelength close to the wavelength for DNA absorption at 260 nm, namely UV-B, causes a structural modification in the base regions of DNA. The modification includes two types of CPD and 6-4 adduct and both of them are dimers generated through a covalent crosslinking between two pyrimidine (Py) moieties adjacent to each other. When the modification generated in DNA by ultraviolet light is defined as 100%, CPD occupies 70 to 80% and the 6-4 adduct occupies 20 to 30%. These two structures inhibit DNA replication and transcription to cause cellular death and mutagenesis. It is believed that the onset of skin cancer caused by bathing in strong sunlight is triggered by these damages generated by ultraviolet light.
Biological organisms have various repair mechanisms for such damages. Therefore, biological organisms on the earth are not readily develop cancer even when they bath in sunlight. One of the mechanisms is photoreactivation. The repair system allows a photoreactivating enzyme to carry out the reverse reaction to that of Py+Py→CPD caused by ultraviolet light, using the energy of near ultraviolet light and blue light irradiated following ultraviolet light to return the CPD and 6-4 adduct generated by ultraviolet light to former state (1; FIG. 1). As the photoreactivating enzyme carrying out the repair, two types of enzyme exist; one specifically repairs CPD, while the other specifically repairs the 6-4 adduct (2,3). The presence of the CPD photoreactivating enzyme is confirmed widely among prokaryotic organisms and higher eukaryotic organisms. Once the ozone layer is destructed, the dose of ultraviolet light reaching the earth increases. Therefore, it is anticipated that more damages occur in DNA more than ever, leading to the limit of the repair, so that the biological organisms may be influenced by serious harms.
It is true with plants. Biological organisms with no direct need of sunlight can survive while avoiding sunlight, even if the dose of ultraviolet light increases. However, plants getting most of energy via photosynthesis cannot evade sunlight. Consequently, it is estimated that the influence of the destruction of the ozone layer on plants may be more serious than on biological organisms with no direct need of sunlight.
An experiment is reported recently, where Arabidopsis thaliana and Nicotiana were grown in environment at a higher ultraviolet light dose based on a possible decrease of the ozone layer in future as estimated from the current basal value of the dose of ultraviolet light (4). In other words, actual influences on plants were observed in a potential status assumed on the basis of the destruction of the ozone layer. The results are as follows. First, the dose elevation increases the cellular CPD and 6-4 adducts from the current levels, so that their growth is suppressed and their genes are increasingly recombined, leading to the elevation of the instability of the genomes, which involves the increase of the instability in the course of generations. In other words, the elevated ultraviolet light not only influences the generation itself but also gives such influences over some future generations. When the dose irradiated is retained, further, more mutations accumulate in a later generation, so that the generation turns more sensitive to ultraviolet light than preceding generations. This is due to the fact that Arabidopsis thaliana or Nicotiana is more influenced by ultraviolet light because Arabidopsis thaliana or Nicotiana is exposed to the external atmosphere during the term from the dehiscence of the anther as a reproductive organism to the stage of pollination with the pollen of Arabidopsis thaliana or Nicotiana. This is the case with most of plants on the earth. It may be considered that those described about Arabidopsis thaliana and Nicotiana can be induced by the destruction of the ozone layer and the subsequent increase of the dose of ultraviolet light. In other words, this suggests a possibility of the emergence of a severe change in the ecosystem some years after the destruction of the ozone layer.
As described above, it can be said that photoreactivation capable of reducing the influences of ultraviolet light using the energy of visible light supplied by sun in the same manner as for ultraviolet light is a considerably effective ultraviolet protective system for plants hardly capable of avoiding the influences of ultraviolet light in sunlight. A report showing the presence of photoreactivation in higher plants is issued, for supporting those described above.
A report suggests the presence of CPD photoreactivation activity in a higher plant “Oryza” particularly familiar to the Japanese. At the experiment, an appropriate dose of ultraviolet light irradiates the leaf (third leaf) of Oryza, which is subsequently irradiated with visible light (blue light). Then, the amount of CPD in the cells decreases (repairing) in proportion to the duration of visible light irradiation. Depending on the level of visible light irradiated on an individual after germination, additionally, the CPD repair efficiency of the individual was elevated (5). In other words, a larger amount of CPD can be repaired by the same dose of visible light. The CPD photoreactivation activity never similarly occurs in all of Oryza species. An Oryza species (Norin No. 1) with poor ultraviolet resistance is repaired at a slow rate, compared with an Oryza species (Sasanishiki) resistant to ultraviolet light (6). This may possibly be ascribed to the occurrence of some mutation in the photoreactivating enzyme itself in the species with poor ultraviolet resistance or the system regulating the expression. However, the cause has not yet been elucidated.
References and information of the related art in relation with the invention of this application are as follows.    (1) Aziz Sancar. (1994) “Structure and Function of DNA photolyase” Biochemistry 33:2–9.    (2) Takeshi Todo, Hiroshi Takemori, Haruko Ryo, Makoto Ihara, Tsukasa Matsunaga, Osamu Nikaido, Kenji Sato, Taisei Nomura (1993) “A new photoreactivation enzyme that specifically repairs ultraviolet light-induced (6-4) photoproduct” Nature 361:371–374.    (3) Aziz Sancar (1996) “No “End of History” for Photolyases ” Science 272:48–49    (4) Gerhard Ries,Werner Heller,Holger Puchta,Heninrich Sandermann,Harald K.Seidlitz, Barbaara Hohn (2000) “Elevated UV-B radiation reduces genome stability in plants” Nature 406    (5) Hye-Sook Kang, Jun Hidema and Tadashi Kumagai (1998) “Effects of light environment during culture on UV-induced cyclobutyl pyrimidine dimers and their photorepair in rice (Oryza sativa L.)” Photochemistry and Photobiology 68: 71–77    (6) Jun Hidema, Tadashi Kumagai, John C. Sutherland, Betsy M Sutherland (1997) “Ultraviolet B-sensitive rice cultivar deficient in cyclobutyl pyrimidine dimer repair”. Plant Physiology 113: 39–44    (7) Satoshi Nakajima, Munetaka Sugiyama, Shigenori Iwai, Kenichi Hitomi,Eriko Otoshi,Sang-Tae Kim,Cai-Zhong Jiang,Takishi Todo,Anne B Britt, Kazuo Yamamoto (1998) “Cloning and characterization of a gene (UVR3) required for photorepair of 6-4 photoproducts in Arabidopsis thaliana” Nucleic Acid Research 26:638–644    (8) Jason L.Petersen,Darin W.Lang, Gary D.Small (1999) “Cloning and characterization of a class II DNA photolyase from Chlamydomonas” Plant Molecular Biology 40 :110633–1071    (9) Yao-Guang Liu, Kiyotaka Nagaki, Masao Fujita, Kanako Kawaura, Masahiko Uozumi, Yasunari Ogihara. (2000) “Development of an efficient maintenance and screening system for large-insert genomic DNA libraries of hexaploid wheat in a transformation-competent artificial chromosome (TAC) vector.” The Plant Journal 23:687–95.    (10) Kazuo Maruyama, Sumio Sugano (1994) “oligo-capping:a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides” Gene 138:171–174    (11) Michael Herrler (2000) “Use of SMART-generated cDNA for Differential Gene Expression Studies ” Jounal of Molecular Medicine 78:B23.